⁶⁸Ga-PSMA-11 PET, ¹⁸F-PSMA-1007 PET, and MRI for Gross Tumor Volume Delineation in Primary Prostate Cancer: Intermodality and Intertracer Variability

Purpose: To assess the intermodality and intertracer variability of gallium-68 (⁶⁸Ga)- or fluorine-18 (¹⁸F)-labeled prostate-specific membrane antigen (PSMA) positron emission tomography (PET) and biparametric magnetic resonance imaging (bpMRI)-based gross tumor volume (GTV) delineation for focal boosting in primary prostate cancer.

Methods: Nineteen prospectively enrolled patients with prostate cancer underwent a PSMA PET/MRI scan, divided into a 1:1 ratio between ⁶⁸Ga-PSMA-11 and ¹⁸F-PSMA-1007, before radical prostatectomy (IWT140193). Four delineation teams performed manual contouring of the GTV based on bpMRI and PSMA PET imaging, separately. Index lesion coverage (overlap%) and interobserver variability were assessed. Furthermore, the distribution of the voxelwise normalized standardized uptake values (SUV%) was determined for the majority-voted (>50%) GTV (GTV^majority) and whole prostate gland to investigate intertracer variability. The median patientwise SUV% contrast ratio (SUV%-CR, calculated as median GTV^majority SUV% / median prostate gland without GTV^majority SUV%) was calculated according to the tracer used.

Results: A significant difference in overlap% favoring PSMA PET compared with bpMRI was found in the ¹⁸F subgroup (median, 63.0% vs 53.1%; P = .004) but was not present in the ⁶⁸Ga subgroup (32.5% vs 50.6%; P = .100). Regarding interobserver variability, measured Sørensen-Dice coefficients (0.58 vs 0.72) and calculated mean distances to agreement (2.44 mm vs 1.22 mm) were statistically significantly lower and higher, respectively, for the ¹⁸F cohort compared with the ⁶⁸Ga cohort. For the bpMRI-based delineations, the median Sørensen-Dice coefficient and mean distance to agreement were 0.63 and 1.76 mm, respectively. Median patientwise SUV%-CRs of 1.8 (interquartile range [IQR], 1.6-2.7) for ¹⁸F-PSMA and 3.3 (IQR, 2.7-5.9) for ⁶⁸Ga-PSMA PET images were found.

Conclusions: Both MRI and PSMA PET provided consistent intraprostatic GTV lesion detection. However, the PSMA tracer seems to have a major influence on the contour characteristics, owing to an apparent difference in SUV% distribution in the prostate gland.